1. Technical Field
The present invention relates generally to precisely structured polynucleotide molecules, and methods of using the same for multivalent RNA interference and the treatment of disease.
2. Description of the Related Art
The phenomenon of gene silencing, or inhibiting the expression of a gene, holds significant promise for therapeutic and diagnostic purposes, as well as for the study of gene function itself. Examples of this phenomenon include antisense technology and dsRNA forms of posttranscriptional gene silencing (PTGS) which has become popular in the form of RNA interference (RNAi).
Antisense strategies for gene silencing have attracted much attention in recent years. The underlying concept is simple yet, in principle, effective: antisense nucleic acids (NA) base pair with a target RNA resulting in inactivation of the targeted RNA. Target RNA recognition by antisense RNA or DNA can be considered a hybridization reaction. Since the target is bound through sequence complementarity, this implies that an appropriate choice of antisense NA should ensure high specificity. Inactivation of the targeted RNA can occur via different pathways, dependent on the nature of the antisense NA (either modified or unmodified DNA or RNA, or a hybrid thereof) and on the properties of the biological system in which inhibition is to occur.
RNAi based gene suppression is a widely accepted method in which a sense and an antisense RNA form double-stranded RNA (dsRNA), e.g., as a long RNA duplex, a 19-24 nucleotide duplex, or as a short-hairpin dsRNA duplex (shRNA), which is involved in gene modulation by involving enzyme and/or protein complex machinery. The long RNA duplex and the shRNA duplex are pre-cursors that are processed into small interfering RNA (siRNA) by the endoribonuclease described as Dicer. The processed siRNA or directly introduced siRNA is believed to join the protein complex RISC for guidance to a complementary gene, which is cleaved by the RISC/siRNA complex.
However, many problems persist in the development of effective antisense and RNAi technologies. For example, DNA antisense oligonucleotides exhibit only short-term effectiveness and are usually toxic at the doses required; similarly, the use of antisense RNAs has also proved ineffective due to stability problems. Also, the siRNA used in RNAi has proven to result in significant off-target suppression due to either strand guiding cleaving complexes potential involvement in endogenous regulatory pathways. Various methods have been employed in attempts to improve antisense stability by reducing nuclease sensitivity and chemical modifications to siRNA have been utilized. These include modifying the normal phosphodiester backbone, e.g., using phosphorothioates or methyl phosphonates, incorporating 2′-OMe-nucleotides, using peptide nucleic acids (PNAs) and using 3′-terminal caps, such as 3′-aminopropyl modifications or 3′-3′ terminal linkages. However, these methods can be expensive and require additional steps. In addition, the use of non-naturally occurring nucleotides and modifications precludes the ability to express the antisense or siRNA sequences in vivo, thereby requiring them to be synthesized and administered afterwards. Additionally, the siRNA duplex exhibits primary efficacy to a single gene and off-target to a secondary gene. This unintended effect is negative and is not a reliable RNAi multivalence.
Consequently, there remains a need for effective and sustained methods and compositions for the targeted, direct inhibition of gene function in vitro and in vivo, particularly in cells of higher vertebrates, including a single-molecule complex capable of multivalent gene inhibition.